Rubber composition for a tire, based on diene elastomer and a reinforcing titanium oxide

ABSTRACT

The invention provides rubber composition usable for the manufacture of tires, comprising at least (i) one diene elastomer, (ii) one white filler as reinforcing filler and (iii) one coupling agent (white filler/elastomer), the white filler comprising a titanium oxide having the following characteristics:  
     (a) it comprises more than 0.5% by mass of a metallic element, other than titanium, selected from the group consisting of Al, Fe, Si, Zr and mixtures thereof;  
     (b) its specific BET surface area is between 20 and 200 m 2 /g;  
     (c) its average particle size (by mass), d w , is between 20 and 400 nm; and  
     (d) its disagglomeration rate, α, measured by the ultrasound disagglomeration test,  
     at 100% power of a 600-watt ultrasonic probe, is greater than 2×10 −2  μm −1 /s. The invention also provides for the use of a rubber composition according to the invention for the manufacture of rubber articles, in particular tires or semi-finished rubber products intended for these tires. The composition of the invention is particularly useful for the manufacture of colored tires or colored semi-finished articles such as tire treads or sidewalls.

[0001] This application is a continuation of PCT ApplicationPCT/EP00/04682, filed May 23, 2000, now WO 00/73373.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to diene rubber compositions usablefor the manufacture of tires or semi-finished products for tires, inparticular treads for tires, and to reinforcing fillers capable ofreinforcing such rubber compositions.

[0003] In order to reduce fuel consumption and pollution emitted bymotor vehicles, major attempts have been made by tire designers toobtain tires having all of the following characteristics: very lowrolling resistance, improved grip, both on dry ground and on wet orsnow-covered ground, and very good wear resistance.

[0004] Numerous solutions have been proposed to lower the rollingresistance and to improve the grip of tires, but these generally resultin a very great decline in wear resistance. It is known that theincorporation of conventional white fillers such as conventional silicasor aluminas, chalk, talc, titanium oxides, clays such as bentonite orkaolin in rubber compositions used in the manufacture of tires, inparticular treads, results in a reduction in the rolling resistance andan improvement in grip to wet, snow-covered or icy ground. However, suchcompositions also result in an unacceptable decline in wear resistance,because such conventional white fillers do not have a sufficientreinforcement ability in such rubber compositions. Thus, these whitefillers are generally referred to as non-reinforcing fillers or,alternatively, inert fillers.

[0005] One solution to this problem was described in Patent ApplicationEP-A-0 501 227, which discloses a diene rubber composition reinforced bya special precipitated silica (SiO₂) which allows manufacture of a tirehaving substantially improved rolling resistance, without adverselyaffecting the other properties, particularly grip, endurance and wearresistance. European Patent Application EP-A-0 810 258 discloses a dienerubber composition reinforced by another special white filler, aspecific alumina (Al₂O₃) of high dispersibility, which allows productionof tires or treads having an excellent compromise of contradictoryproperties.

[0006] Because of these new white fillers, referred to as reinforcingfillers, it has also been possible to begin commercializing coloredtires, in particular, those with colored treads. For aesthetic reasons,the ability to produce such tires meets a real expectation of users, inparticular for passenger vehicles while, at the same time providing asubstantial saving in terms of fuel.

[0007] Applications WO99/02590 and WO99/06480 disclose rubbercompositions, based on reinforcing silicas or aluminas, used formanufacturing tires having treads or sidewalls of different colors. Oneor more inert fillers (i.e., non-reinforcing ones), such as kaolin, talcor titanium oxide, that function as a pigment or pastelizing agent areused as complementary white fillers in these compositions.

[0008] Among inert fillers, titanium oxides, in particular, arewell-known as white pigments in different matrices such as paints, inks,cosmetics, plastics materials and polymers, such as rubber compositionsintended to be incorporated in tire sidewalls (cf. for exampleCA-A-2054059, CA-A-2058901, CA-A-2228692, GB-A-836716, EP-A-697432, andapplications JP1991/006247, JP1995/149950 and JP1996/059894).

[0009] In addition to their pigmentation or pastelizing properties,titanium oxides also are effective anti-UV agents, that can providebeneficial anti-aging protection of colored rubber compositions. Suchcompositions are normally devoid of carbon black (which is an excellentUV absorber) and thus are sensitive to the degrading action of sunlight(see applications WO99/02590 and WO99/06480 referred to above).Moreover, they cannot be protected by antioxidants (paraphenylenediaminetype) conventionally used in black tires, because of the staining effectof these antioxidants.

[0010] During the course of research, the inventors have found thatthere are special titanium oxides which may be used, not only asanti-aging or pigmentation agents, but as true reinforcing fillerscapable of replacing conventional carbon blacks in rubber compositionsfor tires. These titanium oxides unexpectedly represent an advantageousalternative to the use of reinforcing silicas or aluminas, in rubbercompositions, particularly colored rubber compositions. In other words,compositions can comprise a single filler, a titanium oxide, which hasseveral functions (reinforcement, pigmentation, anti-aging), instead ofseveral fillers where each has a single function.

SUMMARY OF THE INVENTION

[0011] The invention relates to a rubber composition usable for themanufacture of tires, comprising, as base constituents, (i) one or morediene elastomer(s), (ii) a white filler as reinforcing filler and (iii)a coupling agent (white filler/elastomer) linking the reinforcing fillerand the elastomer, wherein the white filler comprises a titanium oxidehaving the following characteristics:

[0012] (a) it comprises more than 0.5% by mass of a metallic elementother than titanium, selected from the group consisting of Al, Fe, Si,Zr and mixtures thereof;

[0013] (b) its specific BET surface area is between 20 and 200 m²/g;

[0014] (c) its average particle size (by mass), d_(w), is between 20 and400 nm; and

[0015] (d) its disagglomeration rate, a, measured by the ultrasounddisagglomeration test, at 100% power of a 600-watt ultrasonic probe, isgreater than 2×10⁻² μm⁻¹/s.

[0016] The prior art neither describes nor suggests the titanium oxideas described above in a rubber composition as reinforcing filler. Theabove titanium oxide is referred to as a “reinforcing titanium oxide”,which, without anything other than an intermediate coupling agent (whitefiller/elastomer) can reinforce rubber compositions usable for themanufacture of tires, in particular treads of tires, having a high wearresistance. In particular, the prior art, e.g., CA-A-2054059,CA-A-2058901 or CA-A-2228692 referred to above, teaches thenon-reinforcing nature of titanium oxides and the necessity of adding areinforcing filler, such as carbon black or silica, to give a minimumlevel of reinforcement to the rubber compositions described.

[0017] Another aspect of the invention is the use of a rubbercomposition according to the invention for the manufacture of rubberarticles, in particular tires or semi-finished rubber products intendedfor such tires, these semi-finished articles selected from the groupconsisting of treads, underlayers intended to be positioned beneath thetreads, crown plies, sidewalls, carcass plies, beads, protectors, innertubes and airtight internal rubbers for tubeless tires. The compositionof the invention is particularly suitable for the manufacture ofsidewalls or tire treads for tires intended to be fitted on passengervehicles, vans, 4×4 vehicles, two-wheelers and heavy vehicles, aircraft,construction machinery, agricultural machinery or handling machinery.The treads may be used in the manufacture of new tires or for recappingworn tires.

[0018] Another subject of the invention are the tires and semi-finishedrubber products themselves comprising a rubber composition according tothe invention.

[0019] A further subject of the invention is the use of a reinforcingtitanium oxide for reinforcing a diene rubber composition usable formanufacturing tires.

[0020] A still further aspect of the invention is a process forreinforcing a diene rubber composition usable for the manufacture oftires, comprising incorporating a reinforcing titanium oxide into thediene rubber composition in the uncured state by thermo-mechanicalkneading.

[0021] The rubber compositions of the invention are particularly suitedfor the manufacture of colored tires or colored semi-finished articles,such as treads or sidewalls. “Colored” rubber compositions, tires orrubber articles in the present description are understood to be rubbercompositions, tires or articles, at least part of which is of a color,including white, other than the conventional black.

DESCRIPTION OF THE DRAWINGS

[0022] The invention and its advantages will be readily understood inthe light of the description and examples of embodiment which follow,and FIGS. 1 to 5 relating to these examples in which:

[0023]FIG. 1 is a diagram of a device suitable for measuring theultrasound disagglomeration rate (α) of a filler in the form ofagglomerates;

[0024]FIG. 2 and FIG. 3 are curves of the change in the size of theagglomerates during ultrasound generation using the device of FIG. 1,for fillers whether or not in accordance with the invention, from whichcurves the disagglomeration rates α are determined; and

[0025]FIG. 4 and FIG. 5 depict curves of the variation of modulus as afunction of elongation for different diene rubber compositions, whetheror not in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0026] I. MEASUREMENTS AND TESTS USED

[0027] I-1. Characterization of the Reinforcing Fillers

[0028] The fillers described herein comprise agglomerates of particles,which may be disagglomerated into particles under the action of anexternal force, for example mechanical working or ultrasound. The term“particle” as used herein is understood to mean an aggregate, and not anelementary particle that may form part of this aggregate. The term“aggregate” is defined herein as an indivisible unit (i.e., which cannotbe cut, divided or shared) which is produced upon synthesis of thefiller, that is generally formed of elementary particles which areaggregated together.

[0029] These fillers are characterized as indicated hereafter.

[0030] a) BET specific surface area:

[0031] The BET specific surface area is determined, in accordance withthe method of Brunauer, Emmet and Teller described in “The Journal ofthe American Chemical Society”, vol. 60, page 309, February 1938,corresponding to Standard AFNOR-NF-T45-007 (November 1987).

[0032] b) Average size of the particles d_(w):

[0033] The average size (by mass) of the particles, d_(w), is measuredafter dispersion of the filler to be analyzed by ultrasounddisagglomeration in an aqueous solution of 0.6% by weight sodiumhexametaphosphate.

[0034] Measurement takes place using a centrifugal X-ray detectionsedimentometer type “XDC” (“X-ray Disk Centrifuge”), sold by BrookhavenInstruments, in accordance with the following method of operation.

[0035] A suspension containing 0.8 g of a sample of filler to beanalyzed in 40 ml of water containing 6 g/l of sodium hexametaphosphateas surfactant is produced by action over 8 minutes at 60% power (60% ofthe maximum position of the output control) of a 1500-watt ultrasonicprobe (Vibracell ¾ inch ultrasound generator sold by Bioblock). Afterultrasound generation, 15 ml of the suspension is introduced into therotating disc. After sedimentation for 120 minutes, the massdistribution of the particle sizes and the average size by mass of theparticles, d_(w), are calculated by the software of the “XDC”sedimentometer (d_(w)=Σ(n₁d₁ ⁵)/Σ(n_(i)d₁ ⁴), where n₁=number of objectsof the size or diameter d₁ class).

[0036] c) Disagglomeration rate α:

[0037] The disagglomeration rate, α, is measured using the “ultrasounddisagglomeration test”, at 100% power of a 600-watt probe. This testmakes it possible to continuously measure the evolution of the averagesize (by volume) of the agglomerates of particles during ultrasoundgeneration, as indicated below.

[0038] The setup used comprises a laser granulometer (“Mastersizer S”,sold by Malvern Instruments, with a He—Ne red laser source, wavelength632.8 nm) and its preparer (“Malvern Small Sample Unit MSX1”), betweenwhich is inserted a continuous-flow treatment cell (Bioblock M72410)provided with an ultrasonic probe (600-watt ultrasound generator,Vibracell ½ inch, sold by Bioblock).

[0039] A small quantity (40 mg) of filler to be analyzed is introducedinto the preparer with 160 ml of an aqueous solution containing 0.5 g/lof sodium hexametaphosphate. The rate of circulation is set at itsmaximum. At least three consecutive measurements are made to determinethe initial mean diameter (by volume) of the agglomerates, referred toas d_(v)[0], in accordance with the known Fraunhofer calculation method(Malvern 3$$D calculation matrix). The ultrasound generation is then setat a power of 100% (“tip amplitude” in maximum position) and theevolution of the mean diameter by volume d_(v)[t] as a function of thetime “t” is monitored for about 8 minutes with one measurementapproximately every 10 seconds. After an induction period (about 3-4minutes), it was noted that the reciprocal of the mean diameter byvolume 1/d_(v)[t] varies linearly, or substantially linearly, with thetime “t” (steady state disagglomeration conditions). Thedisagglomeration rate a is calculated by linear regression of the curveof evolution of 1/d_(v)[t] as a function of the time “t”, within thezone of stable disagglomeration conditions (generally, between about 4and 8 minutes). It is expressed in μm⁻¹/s.

[0040]FIG. 1 shows a diagram of the measuring device used for performingthis ultrasound disagglomeration test. This device comprises of a closedcircuit 1 within which a flow 2 of agglomerates of particles suspendedin a liquid 3 can circulate. This device comprises a sample preparer 10,a laser granulometer 20 and a treatment cell 30. A vent to atmosphericpressure (13, 33), at the level of the sample preparer 10 and of thetreatment cell 30 itself, permits continuous elimination of the airbubbles which form during ultrasound generation (i.e., the action of theultrasonic probe).

[0041] The sample preparer 10 (“Malvern Small Sample Unit MSX1”) isintended to receive the sample of filler to be tested (in suspension inthe liquid 3) and to send it through the circuit 1 at the pre-controlledspeed (potentiometer 17, maximum speed of approximately 3 l/min), in theform of a flow 2 of liquid suspension. Preparer 10 consists of areceiving tank which contains, and through which circulates, thesuspension to be analyzed. It is equipped with a stirrer motor 15 ofvariable speed in order to prevent sedimentation of the agglomerates ofparticles of the suspension. A centrifugal mini-pump 16 is intended tocirculate the suspension 2 in the circuit 1. The entrance 11 to thepreparer 10 is connected to the open air by an opening 13 intended toreceive the sample of filler to be tested and/or the liquid 3 used forthe suspension.

[0042] A laser granulometer 20 (“Mastersizer S”), which continuouslymeasures at regular time intervals, the average size “d_(v)” of theagglomerates as the flow 2 passes, by means of a measuring cell 23, towhich are coupled the automatic recording and calculation means of thegranulometer 20, is connected to preparer 10. Such laser granulometersutilize the principle of light diffraction by solid objects suspended ina medium, the refractive index of which differs from that of the solid.According to the theory of Fraunhofer, there is a relationship betweenthe size of the object and the angle of diffraction of light (thesmaller the object, the greater the angle of diffraction). In practice,it is sufficient to measure the amount of diffracted light for differentdiffraction angles in order to determine the size distribution (byvolume) of the sample d_(v), corresponding to the average size by volumeof this distribution (d_(v)=Σ(n₁d_(i) ⁴)/Σ(n₁d₁ ³), where n₁=number ofobjects of the size or diameter d₁ class).

[0043] A processing cell 30 equipped with an ultrasonic probe 35(converter 34 and probe head 36) intended continuously to break up theagglomerates of particles as the flow 2 passes is inserted betweenpreparer 10 and laser granulometer 20.

[0044] The treatment cell 30 is arranged between the outlet 22 from thegranulometer 20 and the inlet 11 to the preparer 10, such that, inoperation, the flow 2 of particles emerging from the preparer 10 firstpasses through the laser granulometer 20 before entering the treatmentcell 30. This arrangement has two major advantages for measurements.First, the air bubbles due to the action of the ultrasonic probe areeliminated on passing through the preparer 10 (which is in the openair), that is to say, before entering the granulometer 20; theytherefore do not upset the laser diffraction measurement. Second, thehomogeneity of the suspension is improved by first passing through thepreparer 10.

[0045] Moreover, the treatment cell 30 is arranged such that the flow 2of particles which penetrates therein, via an inlet 31, first passes infront of the head 36 of the ultrasonic probe 35. This unconventionalarrangement (the flow 2 entering from the bottom 31 of the cell, and notfrom the top 32) has the following advantages: First, all thecirculating suspension 2 is forced to pass in front of the end 36 of theultrasonic probe 35, which is the zone most active in terms ofdisagglomeration. Second, this arrangement permits initialdegasification after ultrasound generation in the body of the treatmentcell 30 itself, the surface of the suspension 2 then being in contactwith the atmosphere by means of a pipe 33 of small diameter.

[0046] The flow 2 is preferably thermostatically controlled by means ofa cooling circuit 40 arranged, at the level of the cell 30, in a doublecasing surrounding the probe 35, the temperature being controlled, forexample, by a heat sensor 14 immersed in the liquid 3 at the level ofthe preparer 10. The arrangement of the various elements of themeasuring device is optimized so as to restrict as far as possible thecirculating volume, i.e., the length of the connecting tubes (forexample, flexible tubes).

[0047] I-2. Characterization of the Rubber Compositions

[0048] The rubber compositions are characterized, before and aftercuring, as indicated below.

[0049] a) Mooney plasticity:

[0050] An oscillating consistometer such as described in standardAFNOR-NFT-43005 (November 1980) is used. The Mooney plasticity ismeasured in accordance with the following principle: the uncuredcomposition is molded in a cylindrical enclosure heated to 100° C. Afterpreheating for one minute, the rotor turns within the test piece at 2rpm, and the torque used for maintaining this movement is measured afterrotation for four minutes. The Mooney plasticity (MS 1+4) is expressedin “Mooney units” (MU), with 1 MU=0.83 N.m (Newton.meter).

[0051] b) Tensile tests:

[0052] These tests make it possible to determine the elasticity stressesand the breaking properties. Unless indicated otherwise, they arecarried out in accordance with the standard AFNOR-NFT-46002 (September1988).

[0053] The secant moduli at 10% elongation (M10), 100% elongation (M100)and 300% elongation (M300) are measured. These moduli are calculatedreduced to the real section of the test piece and, unless indicatedotherwise, are measured in a second elongation (i.e., after anaccommodation cycle). All these tensile measurements are carried outunder normal conditions of temperature and humidity in accordance withthe standard AFNOR-NFT-40101 (December 1979). Processing the recordedtensile data also makes it possible to trace the curve of the modulus asa function of elongation (see FIG. 4), the modulus used here being thesecant modulus measured in a first elongation (i.e., without anaccommodation cycle), calculated reduced to the real cross-section ofthe test piece.

[0054] c) Hysteresis losses:

[0055] The hysteresis losses (HL) are measured by rebound at 60° C. atthe sixth impact, and are expressed in % in accordance with thefollowing equation:

HL(%)=100[(W ₀ −W ₁)/W ₀],

[0056] where W₀=energy supplied; W₁=energy restored.

[0057] II. CONDITIONS OF CARRYING OUT THE INVENTION

[0058] In addition to the usual additives or those capable of being usedin a sulphur-cross-linkable rubber composition which can be used for themanufacture of tires, the compositions according to the inventioncomprise, as base constituents, (i) one or more diene elastomer(s), (ii)at least one reinforcing white filler and (iii) at least one couplingagent between the filler and the elastomer, said reinforcing fillercomprising a reinforcing titanium oxide as described in detailhereafter.

[0059] II-1. Diene Elastomer

[0060] “Diene” elastomer or rubber is understood to mean, in knownmanner, an elastomer resulting at least in part (i.e., a homopolymer ora copolymer) from diene monomers (monomers bearing two doublecarbon-carbon bonds, whether conjugated or not).

[0061] Generally, “essentially unsaturated” diene elastomer isunderstood to mean a diene elastomer resulting at least in part fromconjugated diene monomers having a content of members or units of dieneorigin (conjugated dienes) which is greater than 15% (mole %).

[0062] Thus, for example, diene elastomers such as butyl rubbers orcopolymers of dienes and of alpha-olefins of the EPDM type do not fallwithin the preceding definition, and may in particular be described as“essentially saturated” diene elastomers (low or very low content ofunits of diene origin which is always less than 15%).

[0063] Within the category of “essentially unsaturated” dieneelastomers, “highly unsaturated” diene elastomer is understood to mean adiene elastomer having a content of units of diene origin (conjugateddienes) which is greater than 50%.

[0064] These definitions being given, the following are understood inparticular to be meant by diene elastomer capable of being used in thecompositions according to the invention:

[0065] (a) any homopolymer obtained by polymerization of a conjugateddiene monomer having 4 to 12 carbon atoms;

[0066] (b) any copolymer obtained by copolymerization of one or moredienes conjugated together or with one or more vinyl aromatic compoundshaving 8 to 20 carbon atoms;

[0067] (c) any ternary copolymer obtained by copolymerization ofethylene, and an α-olefin having 3 to 6 carbon atoms with anon-conjugated diene monomer having 6 to 12 carbon atoms, such as theelastomers obtained from ethylene, from propylene with a non-conjugateddiene monomer of the aforementioned type, including inter alia1,4-hexadiene, ethylidene norbornene or dicyclopentadiene;

[0068] (d) any copolymer of isobutene and isoprene (butyl rubber), aswell as halogenated, in particular chlorinated or brominated, versionsof this type of copolymer.

[0069] Although it applies to any type of diene elastomer, a personskilled in the tire art will understand that the present invention, inparticular when the rubber composition is intended for a tire tread, isused primarily with essentially unsaturated diene elastomers, inparticular those of type (a) or (b) above.

[0070] Particularly suitable conjugated dienes are 1,3-butadiene,2-methyl-1,3-butadiene, the 2,3-di(C1 to C5 alkyl)-1,3-butadienes, suchas 2,3-dimethyl-1,3-butadiene, 2,3-diethyl-1,3-butadiene,2-methyl-3-ethyl- 1,3-butadiene, and 2-methyl-3-isopropyl-1,3-butadiene, an aryl-1,3-butadiene, 1,3-pentadiene and 2,4-hexadiene.

[0071] Suitable vinyl aromatic compounds are, in particular, styrene,ortho-, meta- and para-methylstyrene, the commercial mixture“vinyl-toluene”, para-tertiobutylstyrene, the methoxy-styrenes, thechloro-styrenes, vinyl mesitylene, divinyl benzene and vinylnaphthalene.

[0072] The copolymers comprise between 99% and 20% by weight of dieneunits and between 1% and 80% by weight of vinyl aromatic units. Theelastomers may have any microstructure, which is a function of thepolymerization conditions used, in particular of the presence or absenceof a modifying and/or randomizing agent and the quantities of modifyingand/or randomizing agent used. The elastomers may for example be block,statistical, sequenced or microsequenced elastomers, and may be preparedin dispersion or in solution. They may be coupled and/or starred oralternatively functionalized with a coupling and/or starring orfunctionalizing agent.

[0073] The following are preferred polymers: polybutadienes, inparticular those having a content of 1,2-units between 4% and 80%, orthose having a content of cis-1,4 bonds of more than 80%; polyisoprenes;butadiene-styrene copolymers, in particular those having a styrenecontent of between 5% and 50% by weight and, more particularly, between20% and 40%, a content of 1,2-bonds of the butadiene part of between 4%and 65%, and a content of trans-1,4 bonds of between 20% and 80%;butadiene-isoprene copolymers, particularly those having an isoprenecontent of between 5% and 90% by weight and a glass transitiontemperature (Tg) of between −40° C. and −80° C.; and isoprene-styrenecopolymers, particularly those having a styrene content of between 5%and 50% by weight and a Tg of between −25° C. and −50° C.

[0074] Particularly suitable butadiene-styrene-isoprene copolymersinclude those having a styrene content of between 5% and 50% by weight,particularly between 10% and 40%, an isoprene content of between 15% and60% by weight, and more particularly between 20% and 50%, a butadienecontent of between 5% and 50% by weight, more particularly between 20%and 40%, a content of 1,2-units of the butadiene part of between 4% and85%, a content of trans-1,4 units of the butadiene part of between 6%and 80%, a content of 1,2- plus 3,4-units of the isoprene part ofbetween 5% and 70%, and a content of trans-1,4 units of the isoprenepart of between 10% and 50%, and more generally anybutadiene-styrene-isoprene copolymer having a Tg of between −20° C. and−70° C.

[0075] According to a preferred embodiment of the invention, the dieneelastomer of the composition according to the invention is a highlyunsaturated diene elastomer selected from the group consisting ofpolybutadienes (BR), polyisoprenes (IR), natural rubber (NR),butadiene-styrene copolymers (SBR), butadiene-isoprene copolymers (BIR),butadiene-acrylonitrile copolymers (NBR), isoprene-styrene copolymers(SIR), butadiene-styrene-isoprene copolymers (SBIR) and mixturesthereof.

[0076] When the composition according to the invention is intended foruse in a tire tread, the diene elastomer is preferably abutadiene-styrene copolymer, prepared in solution, having a styrenecontent of between 20% and 30% by weight, a content of vinyl bonds ofthe butadiene part of between 15% and 65%, a content of trans-1,4 bondsof between 20% and 75%, and a glass transition temperature Tg of between−20° C. and −55° C. This butadiene-styrene copolymer may also be used ina mixture with a polybutadiene having preferably more than 90% cis- 1,4bonds.

[0077] In a further embodiment intended for use in a tire sidewall, thecomposition according to the invention may contain at least oneessentially saturated diene elastomer, in particular at least one EPDMcopolymer. This copolymer may be used in a mixture with one or more ofthe highly unsaturated diene elastomers referred to above, or not.

[0078] The compositions of the invention may contain a single dieneelastomer or a mixture of several diene elastomers. The diene elastomeror elastomers may also be used in association with any type of syntheticelastomer other than a diene one, or even with polymers other thanelastomers, such as thermoplastic polymers.

[0079] II-2. Reinforcing Filler

[0080] As used herein, a “reinforcing” white filler is understood tomean a white filler (i.e., an inorganic filler, particularly a mineralfiller, sometimes also called “clear filler”) which is capable, on itsown, without any means other than an intermediate coupling agent, ofreinforcing a rubber composition intended for the manufacture of tires.In other words a reinforcing white filler can replace a conventionalfiller of tire-grade carbon black in its reinforcement function.

[0081] The composition according to the invention is reinforced by areinforcing white filler, comprising a reinforcing titanium oxide havingthe following characteristics:

[0082] (a) it comprises more than 0.5% by mass of a metallic element,other than titanium, selected from the group consisting of Al, Fe, Si,Zr and mixtures thereof;

[0083] (b) its specific BET surface area is between 20 and 200 m²/g;

[0084] (c) its average particle size (by mass), d_(w), is between 20 and400 nm; and

[0085] (d) its disagglomeration rate, α, measured using the ultrasounddisagglomeration test, at 100% power of a 600-watt ultrasonic probe, isgreater than 2×10⁻² μm⁻¹/s.

[0086] “Titanium oxide” is understood herein to mean any compound offormula TiO₂ and its possible hydrated forms, irrespective of itscrystalline structure (e.g., rutile, anatase or a mixture of the twocrystalline varieties). It must satisfy, in combination, all thecharacteristics (a) to (d) above.

[0087] An essential characteristic of this reinforcing titanium oxide isthat it is doped by at least one specific metallic element referred toas a “doping” element. In other words, its reinforcement function isactivated or increased by the presence of this doping element. “Doped”titanium oxide is understood in the present application to mean atitanium oxide, the particles of which comprise, at their surface and/orwithin their mass, more than 0.5% by mass of a metallic element, otherthan Ti, selected from the group consisting of Al, Fe, Si, Zr andmixtures thereof. This metallic doping element may be integrated intothe crystalline structure or lattice of the titanium oxide oralternatively, fixed or deposited in a layer which adheres to thesurface of the titanium oxide particles. In other words, the particlesof the reinforcing titanium dioxide may contain the metallic dopingelement within their structure or at least in part coated thereon. The %by weight indicated is the content of metallic element (atom) determinedin known manner by elemental chemical analysis.

[0088] Below the minimum amount of 0.5%, the titanium oxide acts like anon-doped product and exhibits significantly poorer reinforcementperformance. Thus, the amount of metallic doping element in thereinforcing titanium dioxide (% by weight of reinforcing titaniumdioxide) is preferably higher than 1%.

[0089] The maximum amount, depending on the nature of the dopingelement, may be as high as 10% to 15%, or even more. A person skilled inthe art will readily understand that if the metallic doping element isnot distributed substantially uniformly in the mass of the reinforcingtitanium oxide, it must then be present at least on the surface and/oron the periphery of the titanium oxide particles.

[0090] It is generally known that in order to obtain the optimumreinforcement properties imparted by a filler, it is important for thefiller to be present in the rubber matrix in a final form which is bothas finely divided and distributed as homogenously as possible. Suchconditions can be achieved only if the filler can be readilyincorporated in the matrix during mixing with the elastomer anddisagglomerates in order to be dispersed homogenously in the matrix.

[0091] The intrinsic dispersibility of a filler can be evaluated usingthe ultrasound disagglomeration test described in Section I above, tomeasure its disagglomeration rate α.

[0092] For a rate α greater than 2×10⁻² μm⁻¹/s, the titanium oxide hasgood dispersibility. Only a few micronic agglomerates are observed byreflection in optical microscopy on a section of rubber compositionprepared according to known techniques. To obtain even better dispersionof the reinforcing titanium oxide in the diene rubber matrix and,therefore, optimum reinforcement, the disagglomeration rate α ispreferably greater than 5×10⁻² μm⁻¹/s.

[0093] When the BET surface area of the titanium oxide is less than 20m²/g, the compositions have easier working and a reduced hysteresis, butthere is a decline in the breaking and wear resistance properties in thetire. When the BET surface area is greater than 200 m²/g, working in theuncured state becomes more difficult (higher Mooney plasticity)resulting in reduced dispersion of the filler in the matrix. Excessivelylarge sized particles d_(w), greater than 400 nm, act like defects whichlocalize stresses and are harmful in terms of wear. When size d_(w) istoo small, i.e., less than 20 nm, working in the uncured state and thedispersion of the filler during this working are impaired. Thus, the BETsurface area preferably lies within a range from 30 to 150 m²/g and theparticle size d_(w) preferably lies within a range from 30 to 200 nm.

[0094] When the compositions of the invention are intended for use intire treads having low rolling resistance, the reinforcing titaniumoxides have at least one of the following characteristics, preferablyboth:

[0095] BET surface area within a range of 70 to 140 m²/g;

[0096] particle size d_(w) within a range of 50 to 100 nm.

[0097] Moreover, the reinforcing titanium oxide particles havesufficient surface reactivity, i.e., a sufficient number of surfacehydroxyl (—OH) functions which are reactive with respect to the couplingagent, which is particularly beneficial to the reinforcement functionperformed by the filler, and hence to the mechanical properties of therubber compositions of the invention.

[0098] The physical state of the reinforcing titanium oxide includespowder, microbeads, granules, pellets, spheres or any other densifiedform.

[0099] The reinforcing titanium oxide described above may be used on itsown or in association with one or more other reinforcing filler(s), forexample a second white filler such as a silica or a reinforcing alumina.Preferred reinforcing silicas include dispersible precipitated silica,in particular when the invention is used to manufacture tires having lowrolling resistance. Non-limiting examples of such preferred highlydispersible silicas include the silicas BV3380 and Ultrasil 7000 fromDegussa, the silicas Zeosil 1165MP and 1115MP from Rhône-Poulenc, thesilica Hi-Sil 2000 from PPG and the silicas Zeopol 8715 or 8745 fromHuber. A preferred reinforcing alumina includes a highly dispersiblealumina, such as described in application EP-A-0 810 258. Specificexamples include aluminas A125 or CR125 (from Baïkowski), APA-100RDX(from Condea), Aluminoxid C (from Degussa) or AKP-G015 (SumitomoChemicals).

[0100] The reinforcing titanium oxide, alone or optionally inassociation with a second white filler, also may be used in a blend,i.e., in a mixture, with one or more conventional tire-grade carbonblacks. Suitable carbon blacks include all the carbon blacks, inparticular the carbon blacks HAF, ISAF or SAF, conventionally used intires, particularly in treads. Non-limiting examples of such blacksinclude the blacks N115, N134, N234, N339, N347 and N375. The quantityof carbon black present in the total reinforcing filler may vary withinwide limits, but is preferably less than the quantity of reinforcingwhite filler present in the rubber composition.

[0101] In the compositions of the invention, the reinforcing titaniumoxide preferably constitutes the majority, i.e. more than 50% by weight,of the total reinforcing filler. It may advantageously constitute theentire reinforcing filler.

[0102] Preferably, the amount of total reinforcing filler in thecompositions of the invention lies in a range from 20 to 400 phr (partsper 100 parts by weight of elastomer), more preferably from 30 to 200phr. The optimum differs according to the intended applications. Forexample, the level of reinforcement expected for a bicycle tire is lessthan that required for a tire for a passenger vehicle or for a utilityvehicle, such as a heavy vehicle. When the compositions of the inventionare intended for tire treads, the amount of reinforcing white filler ispreferably between 50 and 150 phr.

[0103] The metallic doping element and mixtures thereof are preferablyAl (aluminium) and/or Si (silicon).

[0104] Non-limiting examples of reinforcing titanium oxides, suitablefor the rubber compositions of the invention, include the titaniumoxides sold as anti-UV agents for varnishes or paints by SachtlebenChemie, under the trade names Hombitec RM300 and RM400 (approximately88% and 78% by weight of rutile TiO₂, respectively).

[0105] The titanium oxide Hombitec RM400 is more particularly preferred,because of its combined characteristics of specific BET surface area,average particle size d_(w) and disagglomeration rate a. This compoundhas been specially developed (supplier's technical documentation) foranti-UV protection of wood and is incorporated in varnishes or paintsfor wood. Its elemental chemical analysis reveals that it comprises (%by weight of reinforcing titanium oxide) aluminium (approximately 14.3%of element Al) and iron (approximately 5.3% of element Fe) as metallicdoping elements.

[0106] II-3. Coupling Agent

[0107] When a reinforcing white filler, such as a silica or areinforcing alumina is used in rubber compositions, it is well-knownthat a (white filler/elastomer) coupling agent, also referred to as abonding agent, should be used, the role of which is to produce a bond(or “coupling”) between the white filler and the elastomer, whilefacilitating the dispersion of this white filler within the elastomericmatrix.

[0108] The reinforcing titanium oxide also requires the use of such acoupling agent in order fully to perform its function of reinforcingfiller in the rubber compositions according to the invention.

[0109] “Coupling” agent (filler/elastomer) is more precisely understoodto mean an agent capable of establishing a sufficient chemical and/orphysical connection between the filler and the elastomer, whilefacilitating the dispersion of the filler within the elastomeric matrix.Such a coupling agent, which is at least bifunctional, has thesimplified general formula “Y-T-X”, in which:

[0110] Y represents a functional group (“Y” function) which is capableof bonding physically and/or chemically with the white filler, such abond being able to be established, for example, between a silicon atomof the coupling agent and the surface hydroxyl (OH) groups of the filler(for example, surface silanols in the case of silica);

[0111] X represents a functional group (“X” function) which is capableof bonding physically and/or chemically with the elastomer, for exampleby means of a sulphur atom;

[0112] T represents a hydrocarbon group that links Y and X.

[0113] The coupling agents are not to be confused with simple agents forcovering the white filler which, in known manner, comprise the Yfunction which is active with respect to the filler but are devoid ofthe X function which is active with respect to the elastomer.

[0114] Coupling agents, of variable effectiveness, have been describedin a large number of documents and are well-known to the person skilledin the art. In fact, any known coupling agent may be used to ensure, indiene rubber compositions which can be used for the manufacture oftires, the effective bonding or coupling between the white reinforcingfiller, e.g., titanium oxide, and diene elastomer. These coupling agentsinclude organosilanes, in particular polysulphurized alkoxysilanes ormercaptosilanes, or polyorganosiloxanes bearing the X and Y functionsdescribed above.

[0115] In particular, polysulphurized alkoxysilanes such as described inU.S. Pat. Nos. 3,842,111; 3,873,489; 3,978,103; 3,997,581; 4,002,594;5,580,919; 5,583,245; 5,663,396; 5,684,171; 5,684,172; and 5,696,197,which describe such known compounds in detail, are used.

[0116] “Symmetrical” polysulphurized alkoxysilanes which satisfy thefollowing general formula (I) are particularly suitable for use in theinvention, without the definition below being limiting:

(I) Z—A—S_(n)—A—Z,

[0117] in which:

[0118] n is an integer from 2 to 8 (preferably 2 to 5);

[0119] A is a divalent hydrocarbon (preferably a C₁-C₁₈ alkylene or aC₆-C₁₂ arylene, more preferably a C₁-C₁₀, alkylene, still morepreferably a C₂-C₄, alkylene, in particular propylene); and

[0120] Z corresponds to one of the formulae below:

[0121] in which:

[0122] R¹, which may or may not be substituted, and may be identical ordifferent, represents a C₁-C₁₈ alkyl group, a C₅-C₁₈ cycloalkyl group,or a C₆-C₁₈ aryl group (preferably a C₁-C₆ alkyl group, a cyclohexyl ora phenyl group, in particular a C₁-C₄ alkyl group, more particularlymethyl and/or ethyl),

[0123] R², which may or may not be substituted, and may be identical ordifferent, represents a C₁-C₁₈ alkoxyl group or a C₅-C₁₈ cycloalkoxylgroup (preferably a C₁-C₈ alkoxyl group or a C₅-C₈ cycloalkoxyl group,more particularly methoxyl and/or ethoxyl).

[0124] In commercially available polysulphurized alkoxysilanes inaccordance with Formula (I) above, in particular mixtures, it will beunderstood that the average value of the “n”s is a fractional number,preferably between 2 and 5.

[0125] Suitable polysulphurized alkoxysilanes include the polysulphides(in particular the disulphides or tetrasulphides) ofbis(alkoxy(C₁-C₄)-silylpropyl), in particularbis(trialkoxy(C₁-C₄)-silylpropyl), in particular polysulphides ofbis(3-trimethoxysilylpropyl) or of bis(3-triethoxysilylpropyl).Preferably bis(3-triethoxysilylpropyl) tetrasulphide, abbreviated TESPT,having the formula [(C₂H5O)₃Si(CH₂)₃S₂]_(2,) orbis(triethoxysilylpropyl) disulphide, abbreviated TESPD, having formula[(C₂H₅O)₃Si(CH₂)₃S]₂ are used. TESPD is sold, for example, by Degussaunder the names Si266 or Si75 (in the latter case, in the form of amixture of disulphide (75% by weight) and polysulphides), oralternatively by Witco under the name Silquest A1589. TESPT is sold, forexample, by Degussa under the name Si69 (or X50S when it is supported to50% by weight on carbon black), or alternatively by Witco under the nameSilquest A1289 (in both cases, a commercial mixture of polysulphideshaving an average value of n which is close to 4).

[0126] A person skilled in the art will be able to adjust the content ofcoupling agent in the compositions of the invention, according to theintended application, the nature of the elastomer used, and the quantityof reinforcing titanium oxide, optionally supplemented by any otherwhite filler used as complementary reinforcing filler.

[0127] So as to allow for the differences in specific surface area anddensity of the reinforcing white fillers which may be used, as well asthe molar masses of the coupling agents, it is preferable to determinethe optimum amount of coupling agent, in moles per square meter ofreinforcing white filler, for each white filler used. This optimumamount is calculated from the weight ratio [coupling agent/reinforcingwhite filler], the BET surface area of the filler and the molar mass ofthe coupling agent (referred to as M hereafter), according to thefollowing equation:

(moles/m² white filler)=[coupling agent/white filler](1/BET) (1/M)

[0128] Preferably, the quantity of coupling agent used in thecompositions according to the invention is between 10⁻⁷ and 10⁻⁵ molesper square meter of total reinforcing white filler, i.e., per squaremeter of reinforcing titanium oxide when the latter constitutes the solereinforcing white filler present. More preferably, the amount ofcoupling agent is between 5×10⁻⁷ and 5×10⁻⁶ moles per square meter oftotal reinforcing white filler. Generally, the amount of coupling agent,relative to the weight of diene elastomer, will preferably be between 1and 20 phr, more preferably between 3 and 15 phr.

[0129] The coupling agent may first be grafted (via the “X” function) onto the diene elastomer of the composition of the invention, resulting ina functionalized or “precoupled” elastomer which comprises the free “Y”function for coupling to the reinforcing titanium oxide. Alternatively,the coupling agent could be grafted (via the “Y” function) onto thereinforcing titanium oxide to produce a “precoupled” filler which isthen bonded to the diene elastomer by means of the free “X” functions.

[0130] It is preferred to use the coupling agent in the free (i.e.,non-grafted) state or grafted onto the reinforcing titanium oxide,preferably because of better working (“processability”) of thecompositions in the uncured state.

[0131] II-4. Various Additives

[0132] The compositions according to the invention contain, in additionto the compounds already described, all or part of the constituentsusually used in diene rubber compositions intended for the manufactureof tires, such as plasticizers, pigments, protective agents, such asantioxidants and antiozonants, a cross-linking system based either onsulphur or on sulphur and/or peroxide and/or bismaleimide donors,vulcanization accelerators, vulcanization activators, extending oils,etc. A conventional non-reinforcing white filler, such as clay,bentonite, talc, chalk or kaolin, or even a conventional titanium oxide,which has a known function of pigmentation agent or anti-UV protection,may be optionally associated with the reinforcing white filler of theinvention.

[0133] For producing colored rubber compositions, it is thus possible touse any type of coloring agent known to a person skilled in the art. Thecoloring agent may be organic or inorganic, and soluble or insoluble inthe compositions according to the invention. For example, mineralcolorants, such as powdered metals, in particular powdered copper oraluminium, or various metal oxides, in particular silicates, aluminatesor titanates, iron oxides or hydroxides, and mixed oxides of differentmetallic elements, such as Co, Ni, Al or Zn, may be used. Also, organicpigments, such as indanthrones, diketo-pyrrolo-pyrroles or diazocondensates, or organometallic pigments, such as phthalocyanines, may beused in the compositions. The color of colored rubber compositionsaccording to the invention can thus vary within a wide range, forexample different shades of red, orange, green, yellow, blue oralternatively brown or grey. It is also possible to produce a rubbercomposition without adding a coloring agent, and thus to retain theoriginal color of the reinforcing filler, whether the latter be white orhave been precolored.

[0134] Protection systems such as described in applications WO99/02590and WO99/06480 may be used for anti-aging protection of colored rubbercompositions according to the invention.

[0135] Rubber compositions according to the invention may additionallycontain covering agents (comprising the single function Y) for thereinforcing white filler. Such covering agents provide an improvement inthe dispersion of the white filler in the rubber matrix and a reductionin the viscosity of the compositions, which improves their ease ofworking in the uncured state. Such covering agents include alkylalkoxysilanes (in particular alkyl triethoxysilanes), polyols,polyethers (for example polyethylene glycols), primary, secondary ortertiary amines, hydroxylated or hydrolyzable polyorganosiloxanes, forexample, α,ω-dihydroxy-polyorganosiloxanes (in particularα,ω-dihydroxy-polydimethylsiloxanes).

[0136] II-5. Preparation of the Compositions

[0137] The rubber compositions are prepared using the diene elastomersaccording to known techniques such as by thermomechanical working in oneor two stages in an internal paddle mixer, followed by mixing on anexternal mixer, during which the vulcanization (cross-linking) system isincorporated, as described, e.g., in Application EP-A-0 501 227.

[0138] In a conventional one-stage process, all of the constituents ofthe composition, except the vulcanization system, are introduced into aconventional internal mixer. When the apparent density of the filler islow, it may be preferable to divide the introduction thereof intoseveral parts. The result of the first kneading stage is then taken upon an external mixer, generally an open mill, and then the vulcanizationsystem is added. A second stage may be added in the internal mixer,essentially with the aim of subjecting the mix to complementarythermomechanical treatment.

[0139] The invention relates to the rubber compositions both in theuncured state (i.e., before curing) and in the cured state (i.e., aftercross-linking or vulcanization).

[0140] The compositions according to the invention may be used alone orin a blend with any other rubber composition usable for manufacturingtires.

[0141] III. EXAMPLES OF EMBODIMENT OF THE INVENTION

[0142] III-1. Fillers Used

[0143] The characteristics of the fillers used in the following examplesare set forth in Table 1. Fillers A and D are the reinforcing titaniumoxides (Hombitec RM400 and RM300, respectively) used in the compositionsaccording to the invention. Filler B is a conventional (i.e.,non-reinforcing) titanium oxide usually used as a white pigment (PronoxRKB6 from Bayer). Filler C is a tire-grade carbon black (N234) which isconventionally used in tire treads.

[0144] It should be noted that fillers A and D have a particle sized_(w) far lower than that of filler B and of the same order of magnitudeas that of filler C. The disagglomeration rate α of Fillers A and D isvery high, about ten and three times greater, respectively, than that ofFiller B. The aptitude for disagglomeration of carbon blacks is known tobe much greater than that of white fillers, which explains why there isno cause to measure the rate α for Filler C.

[0145] Filler A advantageously satisfies all the preferredcharacteristics of a titanium oxide filler of the invention:

[0146] amount of metallic doping element greater than 1%;

[0147] BET surface area within a range from 70 to 140 m²/g;

[0148] particle size d_(w) within a range from 50 to 100 nm; and

[0149] disagglomeration rate α greater than 5×10⁻² μm⁻¹/s.

[0150]FIGS. 2 and 3 depict the curves of evolution [1/d_(v)(t)=ƒ(t)] ofthe size of the agglomerates, recorded in the ultrasounddisagglomeration test for Fillers A and B, respectively. It can clearlybe seen from FIGS. 2 and 3 (particularly FIG. 3) that the first pointsrecorded (“t” varying from 0 to about 30 s) correspond to themeasurement of the initial mean diameter d_(v)[0], followed (afteractuation of the ultrasonic probe) by progressive passage (here, “t”from 30 s to about 4 min for Filler A) to steady state conditions ofdisagglomeration, during which the reciprocal of “d_(v)” varies linearlywith the time “t”. The recording of the data was stopped here afterabout 8 minutes. The disagglomeration rate α in the zone of steady statedisagglomeration conditions is deduced by elementary calculation oflinear regression, performed by the calculator of the granulometer(between 4 and 8 minutes approximately). A comparison of FIGS. 2 and 3illustrates the superiority of Filler A.

[0151] III-2. Preparation of the Compositions

[0152] The compositions used in the tests are prepared as follows: Thediene elastomer is introduced into an internal mixer having a capacityof 300 ml, filled to 75%, wherein the initial tank temperature isapproximately 90° C. After a suitable kneading time, e.g., 1 minute, allthe other ingredients except the vulcanization system are added,including the filler and associated coupling agent. Thermomechanicalworking is then performed for 10 minutes, with an average blade speed of70 rpm, until a dropping temperature of about 160° C. is obtained.

[0153] The mixture thus obtained is recovered and the vulcanizationsystem (sulphur and a sulphenamide primary accelerator) is added on anexternal mixer (homo-finisher) at 30° C. The vulcanization (curing) iscarried out at 150° C. for 40 minutes.

[0154] III-3. Tests

[0155] a) Test 1

[0156] Three diene rubber compositions intended for the manufacture oftires or treads for tires, in particular for colored tires, are comparedbelow.

[0157] The diene elastomer is an SBR (styrene-butadiene copolymer),prepared in solution, comprising 25% styrene, 58% 1,2-polybutadieneunits and 23% polybutadiene trans-1,4 units.

[0158] These three compositions are identical, except for the following:

[0159] Composition No. 1 (in accordance with the invention): Filler A(with coupling agent);

[0160] Composition No. 2 (not in accordance with the invention): FillerB (with coupling agent);

[0161] Composition No. 3 (not in accordance with the invention): FillerC (without coupling agent).

[0162] The coupling agent TESPT (Si69) was introduced in an amountcorresponding to a surface coverage of approximately 11.6×10⁻⁷ mole/m²of white filler (titanium oxide A or B), where Filler A is a reinforcingfiller and Filler B is not. For carrying out this comparative test, thecoupling agent was also used in control Composition No. 2 whichcomprises only conventional (non-reinforcing) titanium oxide. In thesecompositions, the titanium oxides (Fillers A and B) are used in aniso-volume relative to the carbon black (Filler C).

[0163] Composition No. 3, as a reference, does not require a couplingagent since it is filled with carbon black.

[0164] Table 2 shows the formulation of the different compositions(amount of the different products expressed in phr) and Table 3 showstheir properties before and after curing at 150° C. for 40 minutes. FIG.4 shows the curves of modulus (in MPa) as a function of the elongation(in %). These curves are marked C1 to C3 and correspond to rubberCompositions No. 1 to No. 3, respectively.

[0165] The results show that Composition No. 1, after curing, has alevel of reinforcement which is significantly higher than that ofcontrol Composition No. 2, and substantially equivalent to or evengreater than that of reference Composition No. 3:

[0166] breaking stress about three times higher than Composition No. 2,and of the same order of magnitude as that of Composition No. 3,

[0167] moduli at high deformations (M100 and M300) greater, not onlythan those of Composition No. 2, but also than those of referenceComposition No. 3. The ratio (M300/M100) is significantly higher thanthat of Composition No. 2, and close to that of Composition No. 3,clearly indicating high-quality reinforcement for Composition No. 1according to the invention.

[0168]FIG. 4 confirms the above results. It can be seen that curves C1and C3, which are very close, are located far above curve C2, thedistance becoming more pronounced as the elongation increases. Thisclearly illustrates excellent interaction between the reinforcingtitanium oxide and the elastomer in Composition No. 1, of the same levelas or even greater (visible beyond 200% elongation) than that achievedwith the carbon black (Composition No. 3). In all cases the interactionis very much greater than that offered by conventional titanium oxide inComposition No. 2.

[0169] Compared with reference Composition No. 3, Composition No. 1according to the invention has, in addition to a higher level ofreinforcement, lower hysteresis losses (HL) (26% instead of 32%). Theworking in the uncured state is also more advantageous, due to asubstantially lower level of plasticity (52 MU instead of 60 MU). Thevery low viscosity and hysteresis values of Composition No. 2, can beexplained by a very low level of reinforcement, compared with thatachieved on the Composition No. 1 according to the invention.

[0170] b) Test 2

[0171] The object of this test is the test of a reinforcing titaniumoxide other than that of Test 1, and the demonstration that thereinforcing capacity or lack thereof of such a white filler can berevealed only in the presence of a coupling agent capable of linking thewhite filler and the diene elastomer.

[0172] Three diene rubber compositions similar to those of Test 1 above,intended for the manufacture of tires or treads for tires, are compared.The three compositions are identical, except for the followingdifferences:

[0173] Composition No. 4 (in accordance with the invention): Filler D,with coupling agent;

[0174] Composition No. 5 (not in accordance with the invention): FillerD, without coupling agent;

[0175] Composition No. 6 (not in accordance with the invention): FillerC (without coupling agent).

[0176] In Composition No. 4 according to the invention, the couplingagent (TESPT) was introduced in an amount corresponding to a surfacecoverage of approximately 11.6×10⁻⁷ mole/m² of white filler (titaniumoxide D), as in Test 1 above. The coupling agent was omitted in controlComposition No. 5. Reference Composition No. 6, filled with carbonblack, it is the same reference composition as that used in the previoustest (Composition No. 3).

[0177] Table 4 sets forth the formulation of the different compositions(amount of the different products expressed in phr) and Table 5 providestheir properties before and after curing at 150° C. for 40 minutes. FIG.5 shows the curves of modulus (in MPa) as a function of the elongation(in %). These curves are marked C4 to C6 and correspond to rubberCompositions No. 4 to No. 6, respectively.

[0178] The results show that Composition No. 4, after curing, has alevel of reinforcement which is significantly higher than that of thecontrol Composition No. 5, and substantially equivalent to that of thereference Composition No. 6:

[0179] moduli at the high deformations (M100 and M300) and ratio(M300/M100) significantly higher than those of Composition No. 5, andvery close to, but slightly less than, those of reference CompositionNo. 6;

[0180] better hysteresis (far lower losses HL) for Composition No. 4according to the invention compared with the reference Composition No. 6filled with carbon black, as might be expected owing to the nature ofthe reinforcing filler itself, as well as control Composition No. 5(devoid of coupling agent).

[0181]FIG. 5 confirms the observations above. It can be seen that thecurves C4 and C6, which are very close, are both located far above curveC5.

[0182] All these results clearly demonstrate that the reinforcingcapacity or lack thereof of the titanium oxide (and moreover of anyother white filler) can be revealed only in the presence of a couplingagent capable of linking the white filler and the diene elastomer.

[0183] However, the performance of the reinforcing Filler D (CompositionNo. 4 and curve C4 of FIG. 5), although substantially equivalent to thatof Conventional tire-grade carbon black, and therefore completelysatisfactory for the intended application, is slightly lower than thatof Filler A in Test 1 (Composition No. 1 and curve C1 of FIG. 4). It canbe seen that curve C1 is located beyond curve C3, which is not the casefor curve C4 compared with curve C6. The values of modulus M100 and M300are greater in the case of Filler A. The superior performance of FillerA can be explained by a better compromise of characteristics as far asthe specific BET surface area, the average particle size d_(w) and thedisagglomeration rate α are concerned.

[0184] In summary, the specific titanium oxides of the compositionsaccording to the invention impart highly advantageous properties to thecompositions:

[0185] (1) A reinforcement capacity and wear resistance, for thecompositions containing them which are at least equal, if not superior,to those obtained with carbon black, which were hitherto unknown withtitanium oxides conventionally used in tire rubber compositions of theprior art.

[0186] (2) An advantageous improvement in the resistance to aging and tothe action of UV rays, which is particularly beneficial to theaesthetics and to the retention of the colors in colored rubberarticles, such as colored tires or treads.

[0187] Because of the reinforcing and anti-UV properties of thereinforcing titanium oxides described, the compositions according to theinvention represent an advantageous alternative to the use of rubbercompositions reinforced with white fillers such as reinforcing silicasor aluminas. It is thus possible to manufacture tires having low rollingresistance, or treads intended for such tires, in particular coloredtires or treads, from rubber compositions filled exclusively withreinforcing titanium oxides. TABLE 1 Filler: A B C D He Density (g/ml)3.68 3.85 1.99 3.95 BET surface area (m²/g) 119 9 126 62 d_(w) (nm) 571860 69 78 α (μm⁻¹/s) 0.132 0.013 — 0.036

[0188] TABLE 2 Composition number 1 2 3 SBR (1) 100 100 100 Filler A 92— — Filler B — 96 — Filler C — — 50 Si69 6.85 0.54 — ZnO 2.5 2.5 2.5Anti-ozone wax 1.5 1.5 1.5 stearic acid 2 2 2 antioxidant (2) 1.9 1.91.9 DPG (3) 0.6 0.6 0.6 sulphur 1.5 1.5 1.5 CBS (4) 2.5 2.5 2.5

[0189] TABLE 3 Composition number 1 2 3 Plasticity (MU) 52 28 60 M10(MPa) 8.8 3.1 6.4 M100 (MPa) 7.3 3.0 6.4 M300 (MPa) 18.3 4.6 16.9M300/M100 2.5 1.5 2.6 Breaking stress (MPa) 22.1 7.4 24.1 HL (%) 26 1032

[0190] TABLE 4 Composition number 4 5 6 SBR (1) 100 100 100 Filler D 9999 — Filler C — — 50 Si69 3.9 — — ZnO 2.5 2.5 2.5 Anti-ozone wax 1.5 1.51.5 stearic acid 2 2 2 antioxidant (2) 1.9 1.9 1.9 DPG (3) 0.6 0.6 0.6sulphur 1.5 1.5 1.5 CBS (4) 2.5 2.5 2.5

[0191] TABLE 5 Composition number 4 5 6 Plasticity (MU) 54 62 60 M10(MPa) 6.9 4.8 6.2 M100 (MPa) 6.6 2.5 6.4 M300 (MPa) 15.9 3.8 17.1M300/M100 2.4 1.5 2.7 Breaking stress (MPa) 20.4 19.9 23.7 HL (%) 23 3632

1. A rubber composition usable for the manufacture of tires, comprising,as base constituents, a diene elastomer, a white filler as reinforcingfiller and a coupling agent (white filler/elastomer) that links thereinforcing filler and the elastomer, the white filler comprising atitanium oxide having the following characteristics: (a) it comprises bymass more than 0.5% of a metallic element, other than titanium, selectedfrom the group consisting of Al, Fe, Si, Zr and mixtures thereof; (b)its specific BET surface area is between 20 and 200 m²/g; (c) itsaverage particle size (by mass), d_(w), is between 20 and 400 nm; and(d) its disagglomeration rate, α, measured by the ultrasounddisagglomeration test, at 100% power of a 600-watt ultrasonic probe, isgreater than 2×10⁻² μm⁻¹/s.
 2. The composition according to claim 1 ,wherein the total quantity of reinforcing filler is between 20 and 400phr (parts by weight to one hundred parts of elastomer).
 3. Thecomposition according to claim 1 , wherein the BET surface area of thetitanium oxide is within a range of 30 to 150 m²/g.
 4. The compositionaccording to claim 1 , wherein the average particle size d_(w) of thetitanium oxide is within a range of 30 to 200 nm.
 5. The compositionaccording to claim 1 , wherein the disagglomeration rate α of thetitanium oxide is greater than 5×10⁻² μm⁻¹/s.
 6. The compositionaccording to claim 1 , wherein the reinforcing white filler comprisesmore than 50% by weight titanium oxide.
 7. The composition according toclaim 1 , wherein the total reinforcing white filler is titanium oxide.8. The composition according to claim 1 , wherein the reinforcing whitefiller further comprises silica and/or alumina.
 9. The compositionaccording to claim 1 , further comprising one or more carbon blacks as areinforcing filler.
 10. The composition according to any of claims 1 and9, wherein the quantity total of reinforcing filler is between 30 and200 phr.
 11. The composition according to claim 1 , wherein the quantityof coupling agent is between 10⁻⁷ and 10⁻⁵ mole per square meter ofreinforcing white filler.
 12. The composition according to claim 11 ,wherein the quantity of coupling agent is between 5×10⁻⁷ and 5×10⁻⁶moles per square meter of reinforcing white filler.
 13. The compositionaccording to claim 1 , wherein the titanium oxide satisfies one or bothof the following characteristics: its BET surface area is within therange of 70 to 140 m²/g; its particle size d_(w) is within the range of50 to 100 nm.
 14. The composition according to claim 1 , wherein thetitanium oxide satisfies all the following characteristics: it comprisesby mass more than 1% of a metallic element other than titanium, selectedfrom the group consisting of Al, Fe, Si, Zr and mixtures thereof, itsBET surface area is within the range of 70 to 140 m²/g; its particlesize d_(w) is within the range of 50 to 100 nm; and its disagglomerationrate α is greater than 5×10⁻² μm⁻¹/s.
 15. The composition according toclaim 1 , wherein the coupling agent is a polysulphurized alkoxysilane.16. The composition according to claim 1 , wherein the diene elastomeris selected from the group consisting of polybutadienes, polyisoprenes,natural rubber, butadiene-styrene copolymers, butadiene-isoprenecopolymers, butadiene-acrylonitrile copolymers, isoprene-styrenecopolymers, butadiene-styrene-isoprene copolymers, and mixtures thereof.17. The composition according to claim 16 , wherein the diene elastomeris a butadiene-styrene copolymer prepared in solution having a styrenecontent of between 20% and 30% by weight, a content of vinyl bonds ofthe butadiene part of between 15% and 65%, a content of trans-1,4 bondsof between 20% and 75% and a glass transition temperature of between−20° C. and −55° C.
 18. The composition according to claim 17 , furthercomprising a polybutadiene having more than 90% cis-1,4 bonds.
 19. Thecomposition according to claim 1 , wherein the diene elastomer is anEPDM copolymer.
 20. A reinforcing filler comprising a titanium oxidehaving the following characteristics: (a) it comprises more than 0.5% bymass of a metallic element other than titanium, selected from the groupconsisting of Al, Fe, Si, Zr and mixtures thereof; (b) its specific BETsurface area is between 20 and 200 m²/g; (c) its average particle size(by mass), d_(w), is between 20 and 400 nm; and (d) its disagglomerationrate, α, measured by the ultrasound disagglomeration test, at 100% powerof a 600-watt ultrasonic probe, is greater than 2×10⁻² μm⁻¹/s, whereinthe filler reinforces a diene rubber composition usable formanufacturing tires.
 21. A process for reinforcing a diene rubbercomposition usable for the manufacture of tires, comprisingincorporating by mechanical kneading into the diene rubber compositionin an uncured state a titanium oxide having the followingcharacteristics: (a) it comprises more than 0.5% by mass of a metallicelement, other than titanium, selected from the group consisting of Al,Fe, Si, Zr and mixtures thereof; (b) its specific BET surface area isbetween 20 and 200 m²/g; (c) its average particle size (by mass), d_(w),is between 20 and 400 nm; and (d) its disagglomeration rate, α, measuredby the ultrasound disagglomeration test, at 100% power of a 600-wattultrasonic probe, is greater than 2×10⁻² μm⁻¹/s.
 22. A rubber articlecomprising a composition according to claim 1 .
 23. A tire comprising arubber composition according to claim 1 .
 24. A colored tire comprisinga rubber composition according to claim 1 .
 25. A tread for a tirecomprising a rubber composition according to claim 1 .
 26. A coloredtread for a tire comprising a rubber composition according to claim 1 .